1. Field of the Invention
The disclosed technology generally relates to a sensor and methods for making and using the same, and more particularly relates to a sensor configured to sense the presence of at least one fluidum.
2. Description of the Related Technology
High electron mobility transistors (HEMT) utilize a two-dimensional electron gas (2DEG) formed at a heterojunction as conductive channels. HEMT devices find many applications due to the high mobility of the electrons in the 2DEG. For example, HEMT devices can be used as a sensor for sensing a fluidum. As used herein, a fluidum refers to a state of matter that is not a solid state, and includes gas and liquid states. The article “Development of GaN-based Micro Chemical Sensor Nodes,” by Nicholas Prokopuk, Kyung-Ah Son, Thomas George and Jeong S. Moon, published in IEEE Sensors (2005), for example, describes a sensor for sensing the presence of at least one fluidum (a gas) in an adjoining space. The sensor comprises HEMT with a 2DEG (two-dimensional electron gas) layer stack comprising an AlGaN layer and a GaN layer. A gate overlays at least part of the 2DEG layer stack and electrostatically controls electron density of a 2DEG in the 2DEG layer stack. Molecules interfere with the contact surface of the 2DEG layer stack, in this case the surface of the AlGaN layer, and have an influence on the 2DEG in the 2DEG layer stack. The sensor also comprises a source and a drain electrode for contacting the 2DEG for measuring an electric characteristic of the 2DEG, more in particular the current between the source and the drain.
However, the gate of the sensor for controlling the electron density of the 2DEG must be kept small in order to provide a sufficiently large surface area in between the source and the drain above the 2DEG where fluidum, in this case vapor, molecules can alter an electric characteristic of the 2DEG, such as for example the current in between the source and the drain, for example by being adsorbed by the surface area in between the source and the drain above the 2DEG. As this limits the dimensions of the gate, which is provided for controlling the electron density of the 2DEG, the control of the electron density in the 2DEG is often insufficient.
Moreover, it has been found that the gate, even with limited dimensions, still reduces the area of the surface where the molecules can influence the 2DEG therefore limiting the sensitivity of the sensor, especially at very low concentrations of the molecules of the fluidum which is desired to be detected.